Nuts with captive washers have obvious advantages in applications where it is desirable to fasten a nut and a washer to a threaded fastener in an efficient manner. The washer permits clamping forces to be distributed over a larger area while the nut undergoes relative rotational motion with respect to the washer. They clearly obviate the need for separate steps to install a washer and a nut and eliminate the possibility of using the wrong washer-nut combination. They also simplify problems associated with storage of parts.
Nuts with captive washers are known in the prior art for use in applications demanding highly repeatable fastening torque characteristics. An example is a heavy-duty truck wheel nut application, where safety demands that the truck wheels be installed in a highly reliable and repeatable manner. The outstanding performance of these fasteners is due to the fact that the nut body and the washer have well-matched bearing surfaces which allow low-friction rotational motion between the nut and washer.
In the prior art, it is known to manufacture such captive washer wheel nuts from two pieces, the nut body and the washer, with a flange on the nut body being staked after the parts are assembled to make the washer captive. Obviously, this method of manufacture involves both more processing steps and more pieces to be handled.
Gundersen, et. al., U.S. Pat. No. 1,868,415, discloses a method for producing a washer loosely captive to the head end of a screw by breaking away the washer from the screw body. The washer is formed between two grooves perpendicular to the axis of the screw between the screw head and the threaded shank. The groove closer to the screw head has a smaller diameter than the outer diameter of the threaded shank. The washer is broken away by subjecting it to axial forces directed toward the screw head. The washer is then retained by the resulting frusto-conical breakaway surface on the shank of the screw. This breakaway surface extends beyond the shank-facing side of the washer as the washer is moved toward the screw head. Because most washer-nut applications do not allow portions of the nut to extend beyond the load-facing side of the washer, this method is not suitable for producing nuts with captive washers.
Two patents issued to Pummill, U.S. Pat. Nos. 2,380,994 and 2,520,259, disclose nuts with breakaway washers. However, the washers are intentionally caused to become fixed to the nut as the washer is forced toward the nut it is tightened on a bolt. Because of metal deformation which occurs as the washer is forced toward the nut, the washer is permanently and rotationally fixed against the nut.
Tildesley, U.S. Pat. No. 3,727,254, discloses a method of forming an insert bushing including a ring which, upon use of the bushing, breaks away from the body of the bushing and subsequently undergoes metal deformation to become contiguous with and axially captive to the body of the bushing. Until this metal deformation, the ring is not captive to the bushing body.
It is desirable, therefore, to have a method of manufacturing captive washer wheel nuts which can be treated as a single unit the washers are separated from, and made captive to, the nut body.